Safety stop at 15'..........always

[Walter]

On the Padi RDP the depths of 100 fsw and deeper are shaded grey. It states that all safety stops after dives to 100 fsw and deeper are "mandatory".

It may say that, but that doesn't make it so. Safety stops are optional. If the stop is mandatory, it's a decompression stop.

As long as there's more pressure in the tank than the water pressure outside the tank, there's zero chance of getting water into the tank.

True, and as long as the tank is dry inside when the valve is closed, water cannot get into a tank with a closed valve even if the tank is totally empty.

there's a difference between a deco stop and a safety stop. He obviously didn't understand the difference. Which says something about him..

Yes, there is, but I'm not sure you understand the difference either. A safety stop is, by definition, optional. A decompression stop is mandatory.

Modern tables have mandatory safety stops at this depth.

Only the RDP has what it calls "mandatory safety stops." A mandatory safety stop is not a safety stop, it's a decompression stop.

The only difference is the name. One sounds "scary" and makes it hard to market "non-decompression diving." The other doesn't.

Nope. One is optional, the other is not.

Do you see the confusion though, when PADI tells certed divers they are responsible for their own profiles and then they go on a dive vacation and another person insists on taking responsibilty? I think it makes for a grey zone that is fraught with confusion.

There's no confusion, my dear. You are either a thinking person who knows how to dive and accepts responsibility for your own actions or you are a sheep.

I don't know the solution, but I am always careful about giving too many requirements, as a DM, because I think this implies a certain level of responsibilty I can't really assume with a large group of certified divers.

The solution is simple, but it's not going to happen. Stop certifying people who aren't ready to dive. Stop babysitting people on dives.

Oh I agree, everyone is responsible for themselves. However as the guide, I set the ground rules. For example I set a maximum depth, time, and minimum tank pressure to return to the boat. These limits are non-negotiable. If you want to be more conservative that my limits it's fine.

You don't sound like you agree. You would never know if I followed your limits or followed more conservative limits or less conservative limits. I don't follow guides except in very special situations.

 

[miketsp]

Rate of change is usually defined as the change of a parameter per unit time, (Delta P)/(delta t)

The rate of change of pressure is thus constant if the ascent rate is constant.

edit: The relative rate of change is the rate of change divided by the value of the parameter, (Delta P)/(delta t)/P

The concept is formalised with the derivative
Derivative - Wikipedia, the free encyclopedia

Interesting but irrelevant.

Actually it is the percent rate of change that is most important. That's why M_o is the "Surfacing RATIO."

Thal got it right.
What matters is the percentage change and given a constant vertical speed the percentage of pressure change is greatest near the surface.
Think of it in these terms.
Starting at the surface you need to descend 33ft to double your ambient pressure but to double again you need to go to 99ft.

The percentage change importance derives from Haldane's original experiments in which he found that a 2:1 pressure reduction was required for bubble formation.
Although modern deco theory is a little more sophisticated grossly speaking this is still true and explains why you can ascend faster at greater depths and why you should ascend slower the nearer you get to the surface.

 

[TomP]

Maybe you've never run into crusty old divers who start every sentence with "Son" and talk your ear off about how much better diving was "in the old days"

Not to go off topic but i used to hate those "Son..." lines too but now that i'm old enough to be on the sending side I realize just how much fun they are. I still remember my first "Son.." delivery which just happens to be a diving situation.

A few years back some junior achiever Caribbean divemaster challenged my assertion that my last dive was 2 weeks earlier. I'm guessing his logic was something like: Old guy+winter+New Jersey = no diving.

Without even thinking, and to my surprise, I simply raised my trusty USD Rocket fins and said "Son, these fins are older than you are. Believe what you like, my fins and I don't really care".

 

[ClayJar]

Only the RDP has what it calls "mandatory safety stops." A mandatory safety stop is not a safety stop, it's a decompression stop.

I don't have the physical tables in front of me, but the NAUI RGBM tables are in my Nitrox book. Among the rules listed under "The important RGBM table rules are:" on page 61 is the following:

All dives require a 3-minute safety stop at 5 meters (+/- 1 meter)/15 feet (+/- 3 feet).

Of course, the NAUI RGBM tables don't have letter groups, surface interval tables (1 hour required is the only SIT rule), or RNTs. so they don't work at all the same as "standard" tables. The reasoning may be completely different, but there's another required safety stop (unless you were just talking about the phrase itself, "mandatory safety stop", in which case just ignore this whole reply :biggrin.

 

[Walter]

I wasn't aware of that requirement of the NAUI RGBM tables. Thank you for letting me know. OK, so both NAUI's RGBM and PADI's RDP have decompression stops they incorrectly call safety stops.

 

[The Chairman]

I wasn't aware of that requirement of the NAUI RGBM tables. Thank you for letting me know. OK, so both NAUI's RGBM and PADI's RDP have decompression stops they incorrectly call safety stops.

Actually Walter, RGBM tables allow for a DIRECT ASSENT to the surface at any time. However to continue diving and stay within the tables, you must complete the stops. You are using a rather pedantic definition of "mandatory". Mandatory for WHAT? is the real question. I have no problem with the verbiage of either table and at deeper depths, safety stops become more and more important.

 

[peterbj7]

Rate of change is usually defined as the change of a parameter per unit time. The rate of change of pressure is thus constant if the ascent rate is constant.

A complete non sequitur. Your second sentence does not derive from your first. You are assuming pressure is proportionate to depth, which it patently isn't.

If you can't visualise the basic physics just draw a timeline vertically on a piece of paper and mark out minute intervals all the way to the end of the dive (reaching the surface). Against each write down the depth and the ambient pressure, than plot each separately against time on a cartesian graph. "Depth" will produce a straight line, whereas "pressure" will produce a non-linear curve.

This is basic stuff.

I've left out the spurious mathematical symbols. They look impressive but don't add anything.

 

[captain]

Pete,
I agree. If more divers are getting bent using dive computers, it indicates the divers don't understand what the computers are telling them, which tells me the diver was never trained properly in the set up and use of that particular unit. almtree: Bob

Or they weren't given enough training on decompression to understand what the computer is telling them and how to use that information.

 

[Bob]

Or they weren't given enough training on decompression to understand what the computer is telling them and how to use that information.

Agreed. almtree: Bob

 

[raymond phule]

pressure is proportionate to depth, which it patently isn't.

pressure is a linear function of depth. P=d/10+1, d in meters and P in atm.
0 m->P=1 atm
20m ->P=3 atm

If you can't visualise the basic physics just draw a timeline vertically on a piece of paper and mark out minute intervals all the way to the end of the dive (reaching the surface). Against each write down the depth and the ambient pressure, than plot each separately against time on a cartesian graph. "Depth" will produce a straight line, whereas "pressure" will produce a non-linear curve.

Are you kidding? Why dont you draw the lines your self? See the equation for the pressure above.

Lets assume 10m per min and start at 30m. So

t=0, d=30, p=4 atm
t=1, d=20, p=3 atm
t=2, d=1, p =2 atm
t=3 d=0, p=1 atm

Both seems pretty linear to me.

This is basic stuff.

Yes, but it doesn't seem to be very basic for you.

I've left out the spurious mathematical symbols. They look impressive but don't add anything.

It might be better to actually use the impressive symbols. You might get the correct result then.